In color reproduction, it is often necessary to verify the color accuracy of separation negatives used in the printing process. The use of overlay and surprint proofing systems for this application is well known in the literature. Surprint proofing systems have by far received the most acceptance for applications where color assessment is critical. A surprint proof is generated by successively superimposing different colored layers upon each other on a single sheet. The surprint color proofing materials are composed of two primary types: photosensitive precolored sheets and photosensitive colorless sheets whose latent tacky or electrostatic image may be toned with pigments, inks or dyes. Some examples of post-colored proofing materials may be found in U.S. Pat. Nos. 3,060,024; 3,634,087; 3,649,268; 4,174,216; 4,294,909; 4,356,252; 4,600,669; 4,734,356; 4,869,996; DE 3,336,431 A1 and EP 319,098 patent applications. In these systems the image is colored with wet or dry pigment dispersions. In the application of dry powder toners, the dust can be a nuisance and could potentially be hazardous if inhaled, especially when it is applied manually. The liquid toners are typically dispersed in solvents such as Isopar.TM. which has a pungent odor and is considered combustible making the toner difficult to store and dispose of. Since the solvent must be evaporated to form a dry image, a solvent recovery unit may be necessary if regulated governmental agency limitations of permissible volatile organic components are exceeded. Attempts to eliminate these disadvantages are described in U.S. Pat. No. 4,806,451 and EP 365,361 A2 patent application, which describe the process of laminating a precolored sheet to the colorless image and then peeling it apart to produce the colored image. Other dry peel apart systems which incorporate the color layer with the photoactive layer include; U.S. Pat. Nos. 4,923,780; 4,987,051; 4,895,787; 4,910,120; 4,963,462; GB 2,213,950 A, EP 385,466 A2, EP 365,362 A2, and EP 313,191 A2 patent applications. To date, these systems do not provide the image quality which can be achieved by other methods, especially for negative systems.
Other examples of precolored proofing materials include constructions based upon both diazo oxides and photopolymers as the photosensitive element. In U.S. Pat. Nos. 4,482,625 and 4,304,836 two different precolored photopolymer constructions are described which are imaged prior to lamination to the substrate or intermediate sheet. A similar system is described in GB 2,144,867 A which utilizes precolored films sensitized with diazo based polymers. These systems require the imaged films to be laminated in register which is very dependent upon laminating speed, laminating temperature, and dimensional stability of the film and substrate. The accuracy of the registration is limited to smaller format proofs due to the difficulty in retaining perfect alignment of images during the thermal lamination process. This type of system also makes it very difficult to produce several small proofs on a single large sheet (scatter proofing).
U.S. Pat. No. 4,260,673 describes a positive acting presensitized color proofing sheet comprising a carrier sheet having a smooth release surface, diazo oxide sensitized color layer, a diazo oxide photoactive barrier layer and a thermal adhesive. The photoactive barrier layer is soluble in the developing media when exposed to light and provides a means of reducing interaction between the color layer and adhesive. U.S. Pat. Nos. 4,659,642; 4,772,533; and 4,948,693 describe improvements on this construction by eliminating the barrier layer. However, an aqueous highly alkali developer with pH ranges between 12 and 13 are still necessary for good development.
U.S. Pat. No. 3,671,236 describes a negative acting presensitized color proofing sheet comprising of a carrier sheet having a smooth release surface, a diazo sensitized color layer and a nonphotoactive barrier layer upon which may be coated a pressure sensitive adhesive or a thermal adhesive as described in U.S. Pat. No. 4,656,114. The developing media for this system is a 50:50 mixture of n-Propyl alcohol and water. The barrier layer is present as a carrier for the image and to reduce interaction between the adhesive and the photoactive layer. U.S. Pat. Nos. 4,650,738; 4,751,166; 4,808,508; 4,929,532; and 5,008,174; EP 365,356 A and EP 365,357 A2 patent applications describe improvements on this construction by eliminating the need for a barrier layer between the diazo based photoactive layer and the adhesive. The developer used in these systems are aqueous alkali solutions containing greater than 3% surfactant. U.S. Pat. No. 4,596,757 discloses a further improvement on the construction described in U.S. Pat. No. 3,671,236 utilizing a photo monomer system in place of the diazonium binders. Again an aqueous alkali developer is used which contains high concentrations of a surfactant (the example cited used 8.5%). The present invention improves upon the above mentioned constructions by utilizing a photoactive color layer containing a photo-oligomer as the photopolymerizable component and a photopolymerizable barrier layer to provide a proofing element which can be developed in a low solids aqueous developer having a pH of 10.2. This construction allows one to develop the image using a more environmentally compatible effluent since the aqueous developer contains less caustic and less ingredients than those used in the previously described systems.